1. Field of the Invention
This invention relates to electrical switching apparatus and, more particularly, to circuit breakers, such as, for example, arc fault and/or ground fault circuit breakers.
2. Background Information
Circuit breakers are generally old and well known in the art. Circuit breakers are used to protect electrical circuitry from damage due to an overcurrent condition, such as an overload condition or a relatively high level short circuit or fault condition. In small circuit breakers, commonly referred to as miniature circuit breakers, used for residential and light commercial applications, such protection is typically provided by a thermal-magnetic trip device. This trip device includes a bimetal, which heats and bends in response to a persistent overcurrent condition. The bimetal, in turn, unlatches a spring powered operating mechanism, which opens the separable contacts of the circuit breaker to interrupt current flow in the protected power system.
In many applications, a circuit breaker may provide ground fault protection. Typically, an electronic circuit detects leakage of current to ground and generates a ground fault trip signal. This trip signal energizes a shunt trip solenoid, which unlatches the operating mechanism, typically through actuation of the thermal-magnetic trip device.
A common type of ground fault detection circuit is the dormant oscillator detector including first and second sensor coils. The line and neutral conductors of the protected circuit pass through the first sensor coil. The output of this coil is applied through a coupling capacitor to an operational amplifier followed by a window comparator having two reference values. A line-to-ground fault causes the magnitude of the amplified signal to exceed the magnitude of the reference values and, thus, generates a trip signal. At least the neutral conductor of the protected circuit passes through the second sensor coil. A neutral-to-ground fault couples the two detector coils which causes the amplifier to oscillate, thereby resulting in the generation of the trip signal. See, for example, U.S. Pat. Nos. 5,260,676; and 5,293,522.
Recently, there has been considerable interest in also providing protection against arc faults. Arc faults are intermittent high impedance faults which can be caused, for instance, by worn insulation between adjacent conductors, by exposed ends between broken conductors, by faulty connections, and in other situations where conducting elements are in close proximity. Because of their intermittent and high impedance nature, arc faults do not generate currents of either sufficient instantaneous magnitude or sufficient average RMS current to trip the conventional circuit breaker. Even so, the arcs can cause damage or start a fire if they occur near combustible material. It is not practical to simply lower the pick-up currents on conventional circuit breakers, as there are many typical loads, which draw similar currents and would, therefore, cause nuisance trips. Consequently, separate electrical circuits have been developed for generating an arc fault trip signal to respond to arc faults. See, for example, U.S. Pat. Nos. 5,224,006; and 5,691,869. The arc fault trip signal energizes a shunt trip solenoid, which unlatches the operating mechanism, typically through actuation of the thermal-magnetic trip device.
Metal oxide varistors (MOVs) are electrically connected line-to-neutral in miniature arc fault circuit interrupter (AFCI) and/or ground fault circuit interrupter (GFCI) circuit breakers having, for example, one or two poles.
For example, U.S. Pat. No. 5,293,522 discloses a trip circuit for a single-pole circuit breaker including a line-to-neutral varistor, and a varistor for the trip circuit silicon controlled rectifier (SCR).
U.S. Pat. No. 5,260,676 discloses a trip circuit for a two-pole circuit breaker including two line-to-neutral varistors, and a varistor for the trip circuit SCR.
It is known to employ a MOV in parallel with the trip circuit SCR and in series with the solenoid trip coil in the trip circuit of AFCI and/or GFCI circuit breakers, and a MOV between the load and neutral terminals of such breakers. For a miniature circuit breaker, which is rated for operation at 110-120 VACRMS, it is believed that such MOVs are rated at 150 VRMS.
U.S. Pat. No. 5,519,368 discloses a ground fault circuit including a coil assembly, a rectifier, a MOV and an SCR in parallel with the MOV. For a 120 VRMS line voltage, the rating of the MOV is 150 VRMS.
If the neutral is xe2x80x9clostxe2x80x9d (e.g., due to an electrical problem; due to a xe2x80x9cwhitexe2x80x9d neutral wire being disconnected from the power bus) in a single-pole, two-pole or three-phase power system, then the line-to-neutral voltage may rise to 208 or 240 VRMS, thereby causing the line-to-neutral MOV(s) in a circuit breaker to fail (i.e., due to an excessive voltage condition of sufficient duration).
There is room for improvement in circuit breakers and trip circuits for circuit breakers.
The present invention is directed to a circuit breaker, which employs an excessive voltage detection circuit, such as an MOV, across the circuit breaker trip circuit, such as from the trip solenoid coil to a power conductor (e.g., line or neutral), rather than from line-to-neutral. This permits the trip solenoid to block high voltage surges during surge testing and, also, to trip the circuit breaker if a sufficient line-to-neutral voltage of suitable duration is applied to the circuit breaker.
As one aspect of the invention, a circuit breaker comprises a line terminal adapted to receive a line voltage having a nominal voltage value and an excessive voltage value; a load terminal; at least one neutral terminal; separable contacts electrically connected between the line terminal and the load terminal; an operating mechanism for opening and closing the separable contacts; and a trip circuit cooperating with the operating mechanism to trip open the separable contacts, the trip circuit comprising an electromagnetic assembly including a trip coil having a threshold voltage for energization, a first circuit adapted to detect a trip condition associated with the separable contacts and to responsively energize the trip coil with the line voltage, and a second circuit adapted to detect an excessive voltage condition associated with the load terminal and the at least one neutral terminal and to responsively energize the trip coil with at least the threshold voltage.
Preferably, the excessive voltage condition is about 3 times to about two times the nominal voltage value
The nominal voltage value may be about 110 VRMS to about 120 VRMS; the trip threshold voltage of the trip coil may be about 60 VRMS; the excessive voltage condition may be about 208 VRMS to about 240 VRMS; and the second circuit may have a threshold voltage of about 130 VRMS, in order to responsively energize the trip coil with a voltage greater than the threshold voltage.
The second circuit may be a metal oxide varistor or a sidac.
The first circuit may include a fault detection circuit and a silicon controlled rectifier having a gate triggered by the fault detection circuit to energize the trip coil. The second circuit may include a series combination of a diode, a zener diode and a first resistor, with the series combination of the diode, the zener diode and the first resistor being electrically connected in parallel with the silicon controlled rectifier, and with a second resistor being electrically interconnected between the gate of the silicon controlled rectifier and the first resistor.
As another aspect of the invention, a circuit breaker comprises: first and second line terminals; first and second load terminals; at least one neutral terminal; first separable contacts electrically connected between the first line terminal and the first load terminal; second separable contacts electrically connected between the second line terminal and the second load terminal; an operating mechanism for opening and closing the first and second separable contacts; and a trip circuit cooperating with the operating mechanism to trip open the first and second separable contacts, the trip circuit comprising an electromagnetic assembly including a first trip coil, a second trip coil, a first circuit adapted to detect a trip condition associated with at least one of the first and second separable contacts and to responsively energize the first and second trip coils, a second circuit adapted to detect an excessive voltage condition associated with the first load terminal and the at least one neutral terminal and to responsively energize the first trip coil, and a third circuit adapted to detect an excessive voltage condition associated with the second load terminal and the at least one neutral terminal and to responsively energize the second trip coil.
The second and third circuits may be metal oxide varistors or sidacs.
The first circuit may include a fault detection circuit, three pairs of first and second diodes, and a silicon controlled rectifier having an anode, a cathode and a gate triggered by the fault detection circuit, with each of the first and second diodes having an anode and a cathode, with the anodes of the first diodes being electrically connected to define a first node, with the cathodes of the second diodes being electrically connected to define a second node, with the three pairs of diodes defining a third node, a fourth node and a fifth node, respectively. The first trip coil may be electrically connected between the first load terminal and a first one of the third, fourth and fifth nodes. The second trip coil may be electrically connected between the second load terminal and a second one of the third, fourth and fifth nodes. The at least one neutral terminal may be electrically interconnected with a third one of the third, fourth and fifth nodes. The second circuit may be electrically connected between the at least one neutral terminal and the first one of the third, fourth and fifth nodes. The third circuit may be electrically connected between the at least one neutral terminal and the second one of the third, fourth and fifth nodes.